This invention relates to a composition for forming electrically conductive films which are based on an oxide semiconductor and are useful in various fields such as electrophotography, transparent electrodes, antistatic treatment, reflection of heat rays, and surface heat generator.
Transparent films of an oxide semiconductor generally have a high transmittance to visible light, a low electric resistivity, and a good film strength, and they are widely used as conductive films, e.g., as transparent electrodes in liquid crystal display devices, window materials in solar cells, heat ray-reflecting films, and antistatic films.
Typical of such an oxide semiconductor is an indium-tin oxide (abbreviated as ITO), which is a tin-containing indium oxide, or more specifically an indium oxide containing a minor amount of tin oxide.
Transparent conductive films have conventionally been formed by (1) a dry process in which a film of a metal or inorganic compound (particularly ITO or an analogous semiconductor metal oxide) is deposited onto an insulating substrate by means of vacuum deposition, sputtering, or ion plating, (2) a sol-gel process in which a solution of a decomposable metal compound is applied to a substrate and the resulting coating is dried and calcined to convert the compound into a metal oxide, or (3) a coating process in which a paint--or ink-like dispersion of an electrically conductive powder such as an ITO powder in a binder solution is applied to a substrate and dried or baked.
The dry process has been employed most widely in the formation of transparent conductive films. However, it is disadvantageous in that the deposition rate is low and only 40-55% of the feed material is effectively deposited in the formation of ITO films. The amount of unavailable material is further increased when part of the deposited film is removed by etching to form a circuit or other image. Moreover, the dry process requires complicated and expensive equipment.
The sol-gel and coating processes are both free from the above-described disadvantages. Thus, in these processes, large-size films can be formed with a high productivity using relatively simple equipment. Furthermore, an image such as a circuit can be directly formed by means of screen printing so that a substantial part of the feed material can be effectively used.
In the sol-gel process, however, the material for the substrate is limited, since the coating is finally calcined at a high temperature which is usually above 400.degree. C. Therefore, a plastic substrate cannot be used in this process. In addition, the film thickness formed in a single application is so thin that application and subsequent drying steps should be repeated at least several times in order to obtain a film thick enough to exhibit adequate properties, which makes the procedure complicated.
On the other hand, the coating process can be applied to a plastic substrate since it does not involve a calcining step at a high temperature. It is also possible to obtain a film having a sufficient thickness by a single application. However, in order to form a transparent film by the coating process, it is necessary to use an ultrafine ITO powder having an average particle diameter of 0.5 .mu.m or less and preferably 0.2 .mu.m or less. Since such an ultrafine powder has a high tendency toward agglomeration, it is difficult to uniformly disperse the ITO powder in a binder solution. As a result, the resulting dispersion cannot form a transparent conductive film having desirable film properties. Thus, the ITO-containing conductive film formed from the dispersion has a resistivity which is not decreased sufficiently and it has a relatively high haze.
The dispersibility of an ultrafine ITO powder can be improved either by treating the powder with a dispersant before it is dispersed in a binder solution or by adding a dispersant to the binder solution. However, the use of a dispersant is difficult to decrease the resistivity of the resulting film to a desirable level. It is believed that dispersion of ITO particles with the aid of a dispersant causes the binder and the dispersant to be intimately adsorbed by the particles so as to form an insulating layer on the surface of each particle, thereby making it difficult to decrease the resistivity of an ITO powder-containing conductive film.